1. Field of the Invention
The present invention relates to an oxygen concentration sensor for the exhaust gas of an engine. More specifically, the present invention relates to:
detecting the impedance (that is, the alternating-current impedance) or the admittance of the oxygen concentration sensor;
an oxygen-concentration detecting apparatus which detects the resistance of the oxygen concentration sensor; and
detecting the temperature of the oxygen concentration sensor according to its resistance component.
2. Description of Related Art
In recent years, there has been a demand for an increased control accuracy and a demand for a change to lean burning in control of the air-fuel ratio of an engine used in a vehicle. In response to such demands, there has been provided a linear air-fuel-ratio sensor or an oxygen concentration sensor capable of detecting the air-fuel ratio of mixed air supplied to the engine or the concentration of oxygen contained in exhausted gas which varies with the current flowing through sensor linearly over a wide range.
In order to maintain a high detection accuracy of such an air-fuel-ratio sensor, it is necessary to keep the sensor in an activated state. In general, in order to sustain the activated state of the air-fuel-ratio sensor, it is necessary to heat the sensor element by controlling the current supply to a heater attached to the sensor.
Regarding such current-supply control to the heater, there has been developed a conventional technology of implementing feedback control whereby the temperature of the sensor element is detected and adjusted to a desired activation temperature of typically about 700 degrees Celsius(xc2x0 C.). The temperature of the sensor element is referred to simply as an element temperature. As a possible technique to detect the element temperature from time to time, a temperature sensor may be attached to the sensor element. The element temperature is then derived from output by the temperature sensor. With such a technique, however, the sensor will become large in size and its cost will rise.
In order to solve the problem described above, there has been proposed a technique whereby the impedance of the sensor element which is also referred to hereafter simply as an element impedance is detected instead of the element temperature. The element temperature can then be found from the detected element impedance by using a known relation between the element temperature and the element impedance. It should be noted that a result of detection of the element impedance can also be used for, among other purposes, determining the degree of deterioration of the air-fuel-ratio sensor.
With the conventional element-impedance detecting apparatus, however, the element impedance can not be detected accurately in some cases.
Furthermore, as a technique of detecting element impedance of an air-fuel-ratio sensor of the limit-current type for example, a voltage Vneg is applied to a resistance-dominant zone not including a limit-current zone, and a current Ineg flowing through the sensor as a result of the application of the voltage Vneg is measured. The element impedance is then found as a ratio of the voltage Vneg to the current Ineg as follows:
Element impedance=Vneg/Ineg
According to another technique of detecting an element impedance, an applied voltage is lowered or raised, and a decrease or an increase in flowing current resulting from the decrease or the raise in applied voltage is measured. The element impedance (alternating-current element impedance), is determined by the decrease or increase in applied voltage and the decrease or increase in the flowing current.
According to the conventional technologies, however, conversion from an element impedance to an element temperature may cause an error, and the detection accuracy of the element temperature may be compromised. This problem of a poor detection accuracy is explained as follows. A relation between the element impedance and the element temperature shown in FIG. 25 is known. The vertical axis of FIG. 25 represents the impedance count value obtained as a result of LSB conversion of a detected element impedance. As shown in the figure, the impedance count value is not inversely proportional to the element temperature. Particularly, in the zone of low element temperatures, the impedance count value increases abruptly as the element temperature decreases.
In the activation-temperature zone, on the other hand, a difference of 1 count in impedance count value corresponds to a large difference in element temperature. In an activation-temperature zone of the sensor element, for example, the following relations between the impedance count value and the element temperature exist. When the impedance count value changes from 26 to 25, the derived element temperature substantially changes from 746 degrees Celsius to 750 degrees Celsius. Likewise, when the impedance count value changes from 25 to 24, the derived element temperature greatly changes from 750 degrees Celsius to 760 degrees Celsius. Accordingly, such large change in the element temperature causes a problem that the element temperature is not detected accurately.
The present invention is made in light of the foregoing problems, and it is an object of the present invention to provide an element-resistance-component detecting apparatus for an oxygen concentration sensor and an oxygen-concentration detecting apparatus, having a simple configuration capable of detecting the element resistance component of an oxygen concentration sensor with a high degree of accuracy.
It is another object of the present invention to provide an element temperature detection apparatus for a gas concentration sensor, which can improve the accuracy of element-temperature detection.
According to an aspect of the present invention, an applied-voltage changing means changes a voltage output of a voltage applying means in order to detect the element resistance component of an oxygen concentration sensor. The voltage applying means applies a voltage to the oxygen concentration sensor for detecting the concentration of oxygen in an object gas. Before the applied-voltage changing means changes the oxygen-concentration-detection voltage output by the voltage applying means, a first current detecting means detects a current flowing through the oxygen concentration sensor.
Then, when a predetermined period of time has elapsed after the applied-voltage changing means starts the operation to change the oxygen-concentration-detection voltage outputted by the voltage applying means, a second current detecting means detects the current flowing through the oxygen concentration sensor. Subsequently, a resistor-element calculating means calculates an element resistance component of the oxygen concentration sensor from a difference between a current detected by the first current detecting means and a current detected by the second current detecting means.
That is, the element resistance component of the oxygen concentration sensor is calculated from a difference between a current flowing through the oxygen concentration sensor before changing the oxygen-concentration-detection voltage and a current which flows through the sensor after the predetermined period of time has elapsed after the start of the operation to change the oxygen-concentration-detection voltage.
The voltage applying means includes:
an output circuit for outputting a voltage supplied to an input terminal of the output circuit to the oxygen concentration sensor;
a constant-voltage circuit for supplying a first predetermined constant voltage, to the input terminal of the output circuit through an output resistor; and
a voltage switching circuit having a voltage dividing resistor and a switching device, which are connected in series between a signal line from the output resistor to the input terminal of the output circuit and a first terminal having a second predetermined voltage.
Thus, if the switching device employed in the voltage switching circuit in the configuration described above is put in a turned-off state, the first predetermined constant voltage generated by the constant-voltage circuit is outputted to the oxygen concentration sensor. If the switching device employed in the voltage switching circuit is put in a turned-on state, on the other hand, a divided voltage is outputted to the oxygen concentration sensor. The divided voltage is obtained by dividing the voltage difference between the first predetermined constant voltage and the second predetermined voltage with a ratio of the resistance of the voltage dividing resistor to the resistance of the output resistor.
The applied-voltage changing means changes the oxygen-concentration-detection voltage, that is, the voltage applied to the oxygen concentration sensor, by switching the switching device.
According to the resistance component detecting apparatus for the oxygen concentration sensor of the present invention, when the applied-voltage changing means changes the oxygen-concentration-detection voltage output by the voltage applying means by switching the switching device, the oxygen-concentration-detection voltage output by the voltage applying means changes quickly. Thus, the second current detecting means is capable of detecting the sensor current accurately when a predetermined period of time has elapsed after the oxygen-concentration-detection voltage applied to the oxygen concentration sensor starts changing. As a result, by virtue of the element resistance component of the oxygen concentration sensor, it is possible to change the oxygen-concentration-detection voltage applied to the oxygen concentration sensor without a delay and, hence, to detect the element resistance component of the sensor with a high degree of accuracy with a simple configuration.
According to another aspect of the present invention, the oxygen-concentration-detection voltage applied to the sensor element of the oxygen concentration sensor is temporarily increased or decreased, and an element resistance is calculated as an admittance from a voltage changing amount and a current changing amount caused by such voltage change. Then, the element temperature is determined based on the calculated admittance.
Accordingly, in the detection of an element resistance, the admittance Y, that is, the reciprocal of an element impedance, is calculated as follows:
Y=xcex94I/xcex94V
where notation xcex94I is a change in current and notation xcex94V is a change in voltage. In this specification, by the way, the technical term xe2x80x9celement resistancexe2x80x9d is used to include xe2x80x9celement impedancexe2x80x9d and xe2x80x9celement admittancexe2x80x9d.
Thus, an element resistance is determined from the admittance which has a proportional relationship with element temperature. Accordingly, the element temperature is determined by the admittance, and thereby improving the detection accuracy.